The present invention relates to a process for producing a thin film comprising at least one monomolecular layer of non-amphiphilic molecules.
It is pointed out that "amphiphilic" or "amphipathic" molecules are organic molecules having a hydrophobic part, i.e. a part having a repulsion for polar liquids such as water, and a hydrophilic part, i.e. a part having an affinity for polar liquids such as water.
As a result of these special characteristics, when amphiphilic molecules are placed on the surface of a liquid such as water, the molecules spread on the surface of the liquid and are oriented in such a way that their hydrophilic part is immersed in the water, whilst their hydrophobic part, generally constituted by a hydrocarbon chain, tends to move away therefrom, so that the axis of the chain becomes perpendicular to the water surface. If the cohesion forces between the molecules are adequate, these molecules remain grouped and limit their spread to a continuous monomolecular film having essentially the thickness of a molecule, which corresponds to a Langmuir film. Such films can be manipulated on the water surface and then deposited on a solid support after being compressed under an appropriate surface pressure, in accordance with the Langmuir Blodgett method described in J. of Am. Chem. Soc., vol. 57, 1935, pp. 1007-1010.
It is possible to use this method for depositing several monomolecular layers of the same or different type on a support. In this case the molecules of each of the layers all have the same orientation and films constituted by a group of such layers have interesting uses in electricity, electronics, etc., due to their special structure and their organization.
However, the formation of films from monomolecular layers using the Langmuir Blodgett method involves a special constitution of the molecules to be deposited namely the presence of hydrophobic and hydrophilic parts distributed in the molecule in such a way as to give only a single orientation thereto, when said molecules are dispersed on the surface of a polar liquid, such as water.
Moreover, it is not possible to use the Langmuir Blodgett method when it is wished to form films from non-amphiphilic molecules, i.e. molecules which cannot be oriented on the surface of a polar liquid, and other presently known methods do not make it possible to produce films constituted by monomolecular layers of non-amphiphilic molecules.
Thus, it is sometimes possible to organise non-amphiphilic molecules, i.e. polymer molecules, in the form of a monomolecular layer on the surface of a liquid, but it is not possible to transfer this layer to a support.
In the same way, condensation methods consisting of vacuum evaporation of the non-amphiphilic molecules to be deposited and then the condensation thereof on a support do not make it possible to bring about the deposit of molecules organised in the form of successive laminars parallel to the support. Thus, these methods only give a statistical deposit, and only supply a continuous polycrystalline film in the case of a deposit thickness of several hundred .ANG..
Adsorption methods consisting of bringing a support into the presence of non-amphiphilic molecules to be deposited and dispersed in a gas or dissolved in a solvent only make it possible to deposit a single complete layer, because the following layers are deposited statistically and have a precarious stability. Insertion methods consisting of inserting molecules between the laminars of a host material, such as graphite, can only be used with certain molecules and certain solid host materials. Moreover, the disengagement between the planes containing the inserted molecules is very poor, due to the thinness of the host laminars, which generally corresponds to an atomic monomolecular layer, which prevents certain subsequent uses of the inserted molecules.